1. Field of the Invention
The present invention relates to a wheeled carrier and more particularly to a wheeled carrier with a deployable steering system comprised of a pair of steering wheels centrally located beneath the carrier.
2. BACKGROUND OF THE INVENTION
There is a continuing effort to improve the steering, that is, the tracking and maneuverability of unpowered wheeled carriers such as hospital beds, stretchers, etc. Typically, such carriers are rectangular in shape and contain castors, that is, swiveling wheels, located at the four corners. Such a wheeled carrier is difficult to handle along straight paths because the axes of the pivoting wheels are not maintained in any predetermined fixed relationship. Since the carrier will want to move in the direction of the rotation of a wheel, if the castors are pointed in different directions, the carrier will be pulled in those respective directions, and therefore the carrier will not have any fixed and predictable direction of motion. It is desirable that an operator be able to establish and maintain the path of motion of a carrier. For purposes of this specification, tracking will be defined as the tendency of a carrier to maintain an existing path of motion absent an operator force intended to cause the carrier to deviate from the existing path of motion.
Further, to steer or maneuver a carrier on castors around corners is difficult because there is no fixed pivot axis for turning the carrier. Therefore, the person(s) steering the carrier must, through significant effort, force the carrier to turn as desired.
In order to improve the tracking or maneuverability of such carriers, it is known to deploy one or more additional wheels. For example, as shown in U.S. Pat. No. 2,599,717, a hospital bed with a pair of castors at each end further has a pair of wheels located at the sides which may be lowered thereby lifting one of the pair of castors from the surface. The bed has improved handling for the transportation of a patient.
U.S. Pat. No. 3,304,116, discloses a deployable fixed axis fifth wheel located at the midpoint of a carriage which helps overcome the tendency of the carriage to drift sideways while the carriage is positioned for loading and unloading an object therefrom.
Other designs to facilitate handling include mechanisms to selectively brake one or more castors or to lock castors in a desired position after they have been manually adjusted to that position. Generally, because of the unpredictability of motion and the physical effort required to maneuver prior art carriers, two people are required to steer the carrier. While some designs have a single center wheel to assist turning, such designs have the disadvantage of not improving the tracking of the carrier. Further, other designs may be useful for improving tracking; but they have the disadvantage of being more difficult to turn the carrier through a minimum turning radius, that is, turning the carrier about a pivot point defined by the midpoint of the carrier.